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I recently got a puncture and when I went to replace the inner tube I discovered that I'd bought the wrong size tube. My tyre is 700 x 40c, the tube is 700 x 28-35c.

What is likely to happen if I use this tube? Will the tube explode when I attempt to inflate it?

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  • This is only worth a comment because the question's already been answered correctly. If you are using superlight butyl, latex, or thermoplastic (TPU, e.g. Tubolito) tubes, I'd be more wary of stretching the size specs. For light butyl, you're stretching the tube more than it was designed for if you use an undersized tube, and this might compromise the structural integrity. Latex may be able to stretch, but it is somewhat porous, so I'd be a bit more cautious here. I'm not sure if the material properties of TPU allow this, and those tubes are expensive.
    – Weiwen Ng
    Commented Apr 20, 2021 at 12:42
  • I would also add to all these answers that if the narrower tube is paired with cheap tires that you will have to be topping off their pressure more often because air can more easily permeate out. For going fast, narrower tubes are lighter but with that mindset you might as well buy even smaller tubes and do "ghetto" tubeless.
    – cyanrarroll
    Commented Apr 20, 2021 at 20:05
  • I have been using a 700x38C tire with an 700x23 inner tube inflated to 40-60 PSI and it was very fiddly, constant flats. At least one tube failed at the valve junction and another tube of the same size lost pressure down to not ride worthy in about 2-3 hours even not having and obvious puncture. I solved my issues installing a 28x1.9 inner tube. It holds pressure above 40 PSI for at least 16 days and counting...
    – Jahaziel
    Commented Jan 17, 2023 at 14:22

7 Answers 7

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In my experience smaller ones go in fine and last as per normal (not that I have pushed this to extremes with a 700x20 in a 700x40 but I only have 26x1.5 MTB tubes and they work fine in 26x2.00 tyres).

Allegedly putting a smaller tube in the tyre becomes a problem if and when you repair it, the patch doesn't necessarily stretch with the inner tube and it can come off.

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  • Agreed, no issues with doing this.
    – zenbike
    Commented Jul 5, 2011 at 14:18
  • 4
    A patch just came off my 700x18-23 inner tube in a 700x28 tire due to stretch, so I can confirm this.
    – Sander Heinsalu
    Commented Jan 13, 2016 at 7:06
  • If you patch the tube using a cut out from another inner tube, this solves some of the streching problems. Still not ideal though to over stretch a punctured tube.
    – abdnChap
    Commented Jan 14, 2020 at 11:41
  • An old anecdotal trick I learned from bike-riding, morning rice cake vendors is to put a tube one size larger (in terms of width, not diameter). Same-sized tubes are actually smaller than the tire's space, so a larger tube would have nearly no need to stretch, maintaining its thickness and durability.
    – Gregory Leo
    Commented May 21, 2020 at 18:34
  • @GregoryLeo No, that can cause problems. You are likely to get the inner tube pinched between the bead and the rim while mounting the tire. You would need to be really careful while mounting the tire.
    – rleir
    Commented Jun 21, 2020 at 16:59
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You may be perfectly OK, or may experience several possible problems:

  • The area near the valve on the tube is the stiffest part, and does not expand as easily as the rest when over-inflated. This may result in a slight depression in the tire near the valve that you will feel on every revolution (especially if you start thinking about it). Not a significant reliability issue, but it can be a comfort issue.
  • As the tube expands in its width from over-inflation, it expands even more in its overall circumference. The tube can, before it gets "locked in" to the sidewalls of the tire, expand enough that a portion of the tube telescopes on itself, in the short term creating a lump, and in the only slightly longer term creating a stress point that will result in a fairly sudden deflation of the tire.
  • And obviously, the tube is being placed under more stress than it's designed for, and it's apt to simply fail, especially near the valve.

I wouldn't hesitate to use the (slightly) wrong size tube for a relatively brief period, as an emergency measure, but I wouldn't regard such a repair as trustworthy over the long haul.

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5

Yes, you can, as long as you don't go more than one size smaller, or the differential is not too extreme. A 1.25 in a 1.5 is fine. A 1.25 in a 2.25 is likely to have issues with flats.

Your LBS will have tubes. Are they that bad that you won't even buy tubes from them?

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  • My LBS is a Halfords, they mostly sell car parts and accessories with a small cycling section. They didn't have the right size. My other LBS isn't all that local and other than Saturday is only open when I'm at work.
    – Tom77
    Commented Jul 5, 2011 at 15:11
  • That sucks. But most people really only have the weekend to do what's important. And shopping online for small parts in particular will eventually deprive you of the resource you need when you most need it.
    – zenbike
    Commented Jul 5, 2011 at 17:55
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ANSWER: yes smaller tubes can fill larger tyres, but they may fail quickly and unexpectedly.

I had this exact same situation. I am converting a road bike to urban, and the largest size tire I could fit was a 700x28, the road bike has 700x23.

When I got the old tires off and pulled the tube it was sized 700x23-25. I had changed the tires and reused the old tubes and thought a 10 mile(16 km) ride ought to be safe. 5.2 miles (8.3km) later I was calling my daughter for a pickup. The front tire just went flat. No bump, rock, pothole, nail...just went flat. The rear tire stayed up the entire time, but I am changing its tube too and will keep that one in reserve. I am thinking it was the valve stem.

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Adding to the thoughts above: I had a puncture yesterday on my ride home and used way smaller tube (had the wrong size with me and got another from a fellow bicycle commuter; both tubes were for racing tyre size where mine is 700x33c). First time I tried the tube immediately got damaged and deflated completely - even before I had the wheel back in the frame. The second time it lasted about 100m. So, probably fine to go just one size down but not too much.

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  • 1
    100 metres or miles?
    – rleir
    Commented Jun 21, 2020 at 17:07
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I've had a 19 to 23 tube in a 32mm (700) tyre for a while now and no issues. The 32 is a Michelin Air Max tyre with a very thick sidewall, so that is probably helping.

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My worst experience using a smaller inner tube was putting a 20x1.50-1.75 inner tube into 20x2.125 tire and it resulted in the tire popping off the rim which resulted in permanently damage.

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  • 3
    Why did the tyre come off the rim, because of insufficient pressure inside or did you overpressurise it and cause the bead to fail? Providing an anecdote is not a good as a researched answer, so that should be offset with a careful and complete explanation of what when wrong.
    – DavidW
    Commented Jan 16, 2023 at 16:42

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